In recent years one has become increasingly aware of the impact of human activities on the environment and the negative consequences this may have. Ways to reduce, reuse and recycle resources are becoming more important. In particular clean water is becoming a scarce commodity. Therefore, various methods and devices for purifying water have been published in recent years.
Different methods are known in the art to purify water. For the removal of ions, amongst others hardness ions (Ca2+ and Mg2+) ion-exchange, as disclosed in WO01/30229, or electro-deionisation (EDI), as disclosed in EP-A-1 769 116, are known in the art and commonly used. However, the ion-exchange material as used in these methods require regular regeneration. Further disadvantages of ion-exchange are the limited life-time of the ion-exchange resin and/or the required volume of resin for the production of the amount of soft water, such as in a domestic appliance.
Another known method for water treatment is capacitive deionisation, using a flow through capacitor (FTC) as among others described in U.S. Pat. No. 6,309,532, EP-A-0 861 114, WO02/086195 and WO03/009920. Said method comprises the use of an electrically regenerable electrochemical cell for capacitive deionization and electrochemical purification and regeneration of the electrodes. By charging the electrode, ions are removed from the electrolyte and are held in the electric double layers at the electrodes. The cell can be (partially) regenerated electrically to desorb such previously removed ions. The regeneration could be carried out without added chemical substances.
Flow through capacitors (FTC) generally include one or more pairs of spaced apart electrodes (a cathode and an anode) with current collectors or backing layers provided that are generally adjacent to or very near the electrodes. There is also a flow path for a liquid to travel through the flow-through capacitor and contact the current collectors and electrodes. Current collectors are electrically conductive and transport charge in and out of the electrodes. A conventional FTC comprises a spacer, separating the FTC into a positive charge side and a negative charge side. A high surface area electrode is located adjacent to the spacer and also adjacent to a current collector. In the conventional FTC the layers of electrode, spacer and current collector are fastened together in a “sandwich” fashion by compressive force, normally by mechanical fastening.
In recent publications (U.S. Pat. No. 6,709,560) an improved version of the FTC technology, the so-called charge barrier Flow Through Capacitor technology, is presented, showing that a charge barrier placed adjacent to an electrode of a flow-through capacitor can compensate for the pore volume losses caused by adsorption and expulsion of pore volume ions. The term charge barrier refers to a layer of material which is permeable or semi-permeable and is capable of holding an electric charge. Pore volume ions are retained, or trapped, on the side of the charge barrier towards which the like-charged ions migrate. Generally, a charge barrier functions by forming a concentrated layer of ions. The effect of forming a concentrated layer of ions balances out, or compensates for, the losses ordinarily associated with pore volume ions. This effect allows a large increase in ionic efficiency, which in turn allows energy efficient purification of concentrated fluids.